Blender 2.8 - No theme presets?
Developers have decided that there were too many themes shipped by default with Blender. 

Some were of poor quality, unfinished or not well supported; maintaining them and updating for the new UI is also a lot of work. Developer time is better spent on other features and refining these two to a higher polish level.

Rather than shipping a multitude of themes of dubious quality it is better to focus on two well supported high quality themes instead. One dark and one light, should suffice to please most users taste. 

You can still make your own if you like or download and install third party ones. A call for content has been put up if users want to contribute their own.